The field of the present invention is hardware floor wells.
Wells and particularly oil wells include casings through which perforations are made to allow liquid or gaseous flow into the well from selected strata. Such perforations may be made at various levels within the well for a variety of reasons. At times, however, it has been found advantageous to close off certain perforations within the well. For example, in oil production, due to water flood or steaming activities or simply due to natural causes, perforations at a specific level in the well may be producing excessive water. Perforations at another level may continue to produce oil. Consequently, it is advantageous to close off the perforations producing water. However, as the perforations are displaced from easy reach, a variety of techniques have been employed with varying success and cost.
The most common approach to isolating unwanted casing perforations is to fill the well bore with sand until the unwanted perforations are covered. This sand plug is then capped with a small cement plug to complete the isolation and prevent accidental removal or "bailing" of the sand plug. This method has the advantage of not being a permanent plug as well as being the least expensive. However, such cement caps are often unconsolidated, causing unintentional removal during regular well maintenance work, as well as not effectively isolating the unwanted lower set of perforations. Also the unwanted perforations may be very close to perforations which are to be left unplugged. Such a situation makes placement of the cement cap difficult and the use of this technique has caused desired perforations to be contaminated with cement which can cause a drop in well production. Also under such a circumstance, the rathole available may be reduced to the point that the down hole pump cannot be placed below all perforations and the well work necessary to keep fill from covering perforations is increased significantly.
A second method also widely used to isolate unwanted perforations is to squeeze the perforations with cement. After the perforations have been plugged, the well board is cleaned out which alleviates the problems associated with reducing the rathole. This method has the advantage of allowing perforations to be plugged while leaving perforations both above and below open for production. However, this is a significantly more expensive process than using a cement cap plug on sand. In certain formations, the method is difficult to apply because the formation sands are unconsolidated and have very high permeability. Experience has shown that significant volumes of cement can be pumped without shutting off the perforations. This further increases the cost of this method. Further, the effectiveness of these squeezes is questionable. Difficulties in placing cement in every perforation and the breakdown of the squeeze perforations has caused operating problems and the return to a situation where the perforations were not isolated. Under conditions where high volumes of cement are required to provide this type of plug, it must be considered a permanent abandonment of the zone for that well. Finally, operational problems also occur when the perforated interval to be squeezed is close to an interval to be left open. Also, a significant number of packers have been cemented in place, further contributing to average cost per plug.
Additional alternatives for isolating perforations include casing patches and liner hanger-packers. Both of these methods are prohibitively expensive. Casing patches present a permanent reduction of well bore diameter limiting the use of additional liners at a later time. Further, liner hanger-packers are not designed to be retrieved, making this method semi-permanent.
Rubber seal adapters have been designed which employ a pipe fitting with an annular flange about the periphery thereof. The fitting has external threads at one end to be associated with a liner and internal threads at the other to be associated with a kelly nut. A compression sleeve slidably extends over the pipe fitting. One or more rubber seal rings are positioned between the annular flange and the compression sleeve to be compressed outwardly against the surrounding casing or liner for the isolation of perforations. Floating retainer rings between adjacent rubber seal rings with the annular flange, the floating rings and the compressent ring have undercut portions to receive and retain the rubber seal rings. Such a device is inexpensive to produce and can be retrieved from a well. A positive seal is created and the rathole remains, optimizing pump placement and reducing well work frequency. The effectiveness of the seal is easily tested and should not change with time. The adaptor can be placed very accurately and there is no destruction or contamination of the perforations being plugged. The adaptor is durable, allowing well work without risking damage to the seal. However, the adaptor cannot support tension or compression, consequently the liner must be set on the bottom. This limits the application of such adapters to only those perforations that are below all other perforated intervals. Also, only load pressure differentials are believed to be tolerated.